1. Field of the Invention
The present invention relates to a color image forming apparatus using an electrophotography system.
2. Description of the Related Art
In a color image forming apparatus of an electrophotography system, a so-called tandem system including independent image forming units of respective colors so as to print an image at high speed is known. In such color image forming apparatus, mechanical factors in the image forming units of respective colors cause color misalignment (misregistration) upon superimposing images. Especially, in an arrangement independently including units of a laser scanner (optical scanning device) and photosensitive drum in the image forming unit of each color, steady (to be referred to as DC hereinafter) color misalignment occurs.
In order to correct DC color misalignment, in color misalignment correction control, detection toner images of respective colors are transferred from photosensitive drums onto an image carrier, and relative positions of the detection toner images in a scanning direction and conveyance direction are detected using an optical scanner. However, when detection toner images required to detect DC color misalignment are formed, periodic rotation velocity fluctuations of the photosensitive drums occur due to decentering and the like of rollers required to drive the photosensitive drums and an intermediate transfer belt. Such rotation velocity fluctuations cause non-steady (to be referred to as AC hereinafter) color misalignment, resulting in generation of detection errors.
As a measure against detection errors caused by AC color misalignment, Japanese Patent Laid-Open No. 2001-356542 has proposed the following means. That is, toner marks of respective colors are arranged to be spaced by an integer value at intervals of an integral submultiple of a period of velocity fluctuations as a case of AC components in detection patterns. By averaging mark detection results for respective colors, AC detection errors are eliminated.
FIG. 28 shows an example of the detection patterns of this related art. Assume that a linear mark is used. The number of sets of marks used in averaging processing is n, and a total of eight marks for four colors, that is, traverse line marks tLY1, tLM1, tLC1, and tLK1 and slant line marks sLY1, sLM1, sLC1, and sLK1 are arranged as the first set on the left side. The second and subsequent sets have the similar arrangement, and are repeated up to the n-th set. The same applies to marks arranged on the right side. The right and left detection patterns are formed as an image on a belt in correspondence with positions of right and left optical sensors. Color misalignment in a sub-scanning direction is calculated from detection results of the traverse line marks, and that in a main-scanning direction is calculated from detection results of the traverse line marks and slant line marks.
When the detection patterns as in Japanese Patent Laid-Open No. 2001-356542 are formed, a plurality of sets of detection marks have to be formed, resulting in a large total pattern length and a long color misalignment detection time. Since the total pattern length is large, toner consumption unwantedly increases.
When there are a plurality of periods (for example, a drum period, driving roller period, belt period, and the like) of AC components to be removed in AC color misalignment, detection errors of all the periods cannot be removed, and the detection precision may drop. This is because the total length of detection patterns which can be formed as an image on a belt at the same time is limited to one period of the belt, and the total length of ideal patterns required to remove detection errors of all AC component periods becomes very large beyond the limit normally as one period of the belt. That is, detection patterns formed within one period of the belt cannot remove all AC components, and detection errors remain to some extent.